1. Field of the Invention
This invention relates generally to an apparatus and method for forming a multilayer polymeric web from two or more polymeric film tubes formed from a single blown film extrusion die. More particularly, this invention relates to an apparatus and method for forming and cooling at least pair of molten plastic film tubes exiting a pair of outlet lips of an extrusion die in a blown film process to form a continuous web comprising multiple layers of film.
2. Description of the Related Art
Thin plastic film may be produced, in addition to other manufacturing methods, by extruding plastic material, such as polyethylene resin or other similar polymeric resins, in a process known as blown film extrusion. In a typical blown film extrusion process, resin is fed into a single extruder where an extrusion screw pushes the resin through the extruder. The extrusion screw compresses the resin, heating the resin into a molten state under high pressure. The molten, pressurized resin is fed through a blown film extrusion die typically having a single annular opening or die lip. As the molten material is pushed into and through the extrusion die, a molten plastic film tube emerges from the outlet of the extrusion die. The film tube is fed into a set of nip rollers above the die which pull the tube upwards and collapses tube.
The plastic film tube is blown or expanded to a larger diameter by providing a volume of air within the interior of the tube. The combination of the volume of air and the plastic film tube is commonly referred to as a bubble between the extrusion die and a set of nip rollers. The plastic film tube is commonly cooled by one or more external air rings applying a constant flow of air upward along the outside of the plastic film tube. The ratio between the initial diameter of the tube as it exits the die and its final diameter after it expands is commonly referred to as the blow up ratio (BUR). A number of factors including, but not limited to, the air pressure within the bubble of the plastic film tube, the cooling rate provided by the air ring, the temperature and flow of material out of the extrusion die, and the rate at which the plastic film tube is pulled by the nip rollers impact the blow-up ratio, the ultimate diameter of the plastic film tube, and the ultimate thickness or gauge of the plastic film tube.
As the plastic film tube cools travelling upward toward the nip rollers, the plastic film tube solidifies from a molten state to a solid state after it expands to its final diameter and thickness. The point along the bubble where the plastic film solidifies is known as the frost line. Consequently, the portion of the bubble below the frost line is molten allowing for expansion and thinning of the plastic film tube. Conversely, the portion of the bubble above the frost line has solidified and the diameter and thickness of the plastic film tube is generally fixed at that point. Furthermore, since the film above the frost line has solidified, it tends not to adhere to itself or other films that it may come into contact with.
Improved cooling of the plastic film increases the stability of the bubble, thereby allowing for more accurate control of the physical properties of the plastic film tube, for a broader range of plastic film tube diameters and thicknesses, and improved throughput rates. To facilitate better cooling, it is known in the art to use an internal bubble cooling, or IBC, assembly to dissipate heat from within the interior of the bubble. Without an IBC assembly, the bubble contains a static volume of air applying outward pressure on the plastic film tube but there is no avenue to dissipate the heat absorbed by the static volume of air from the plastic film tube. Consequently, the interior air volume quickly settles at approximately the same temperature as the plastic film tube, providing no cooling benefit. Internal bubble cooling assemblies exchange the warm air within the bubble with cooler air while maintaining a constant pressure. The cooled air within the bubble absorbs heat from the interior surface of the bubble, cooling the plastic film tube more quickly and lowering the frost line for increased bubble stability.
U.S. Pat. No. 7,753,666 issued to Greg Wood on Jul. 13, 2010, and entitled Apparatus and Method for Cooling Plastic Film Tube in Blown Film Process (hereafter, “the Wood Patent”) describes an improved internal bubble cooling assembly using particular air ring assemblies. The internal air ring assemblies improve the flow of the cooled air within the bubble to provide improved bubble stability and allow for improved properties of the plastic film tube. The teachings and specification of the Wood Patent are incorporated herein by reference.
A bubble comprising more than one layer may be formed via blown film extrusion, which is commonly referred to as coextrusion. In coextrusion, more than one discrete annular layer of film is formed within the extrusion die. Typically, each layer is provided to the die from a separate extruder. This enables different polymeric resins with different physical properties to be combined into a single bubble. In typical coextrusion, each layer is combined and placed into intimate contact with each other within the interior of the die and prior to exiting a single annular die lip. Since each layer is in a molten state when combined, the films laminate together, if the resins are compatible with each other.
Coextruded films allow for forming a singular film with multiple laminated layers with the layers bonded to each other on a molecular level. However, it is known that certain material properties of the laminate negatively influence the material properties of the laminate. For instance, the laminate typically inherits the tear strength of the layer with the weakest tear strength. Hence, when tear strength is a critical property for a web, it is undesirable to form a coextruded film with a resin having a low tear strength with another resin having a relatively strong tear strength, even though such a combination may provide other desirable properties. Thus, it would be advantageous for a blown film extrusion process that forms multiple layers of film with distinct properties with adjacent layers in contact with each other, but where the layers are not bonded to each other. With such a process, the layers of the web would maintain their own properties, such as tear strength, and the web could have multiple layers without a layer with a lower tear strength decreasing the tear strength of an adjacent layer.
U.S. Publication No. 2014/0334749 by Michael G. Borchardt “the Borchardt Application”), et. al, entitled Melt-Bonded Thermoplastic Bags with Tailored Bond Strength and Methods of Making the Same, describes forming a bubble with multiple layers with a separate die exit for each layer. The Borchardt Application describes bringing together the molten film from the two layers below at least the frost line of one of the layers such that the inner and outer layers are bonded together. However, The Borchardt Application fails to disclose allowing both bubbles to fully cool below the molten state before coming into contact with each other. Hence, the Borchardt Application requires the utilization of other measures to prevent the physical properties of one layer from degrading the physical properties of the other layer. The Borchardt Application also fails to address the decrease in throughput typical of extruding multiple layers due to the difficulties presented in adequately cooling the multiple tube of polymeric film to maintain an acceptable frost line elevation.
In view of the foregoing, it would be desirable to provide a method of forming a multilayer web from a bubble in bubble process that maintains a throughput rate more typical of high speed commercial operations. It would also be desirable for the method to provide for a multi-layer web where the layers are un-bonded to each other so that each layer maintains its own physical properties, such as tear and dart impact. The present invention addresses these needs.